The present invention relates to a method of converting existing long wet or dry process rotary kiln cement making plants to more energy efficient calcining furnace cement making plants.
In order to manufacture Portland cement, it is generally known that raw material comprising primarily limestone and clay and commonly called meal must first be heated to calcining temperature and then further heated to clinkering temperature to carry out the chemical reactions necessary to turn the meal into cement clinker. Many existing cement plants utilize a long rotary kiln in either a wet process or dry process for carrying out both the calcining and clinkering operations.
It is known to use some form of preheater which utilizes waste heat from the rotary kiln to preheat the raw meal prior to its introduction to the rotary kiln. The use of such a preheater serves to lower fuel consumption and normally allows a reduction in the length of the rotary calcining and clinkering kiln for a given cement making capacity. Typically, a suspension type preheater is used having a plurality of serially connected cyclone separators which provide for raw meal to be alternately introduced into and separated from the flow of hot waste gases from the kiln to thereby preheat the raw meal. Such an apparatus is illustrated in U.S. Pat. No. 2,648,532.
While the combination of a multi-stage suspension preheater with a rotary kiln achieves a higher throughput and lowered overall energy consumption for a cement plant, the multi-stage suspension preheater results in a large pressure drop as heated gases pass through it, causing a large consumption of energy for preheating raw meal. The multi-stage suspension preheater is also formed by several interconnected cyclone separators which are expensive in terms of equipment and installation costs.
A more recent innovation in the cement making process is to utilize a separate, stationary calcining furnace interposed between the suspension preheater and the rotary kiln. U.S. Pat. No. 3,891,382 is typical of such an apparatus. With such an apparatus, the raw meal is preheated in a suspension preheater and substantially completely calcined in suspension in the flash furnace. From the flash furnace, the calcined material is supplied to a rotary kiln where final clinkering takes place. By separating the preheating, calcining and clinkering functions, the quantity of cement clinker which can be produced in a given size rotary kiln can be doubled when compared with a suspension preheater--rotary kiln installation and even more when compared with a long dry or long wet process kiln. In addition, substantial fuel savings can be achieved when compared with long dry or long wet process kilns.
In view of the improved capacity and efficiency of modern plants employing separate preheating, calcining and clinkering stages, it is desirable to modernize older plants which utilize the long dry or wet process of manufacturing cement clinker.
Older long wet or dry process rotary kiln cement making plants can be converted to more efficient cement plant operations by shortening the conventional rotary kiln and utilizing the multi-stage suspension preheater and calcining furnace combination as an input to the remaining portion of the rotary kiln. This technique is described in the article "Conversion of Existing Cement Kilns to Flash Calciners" by J. Warshawsky appearing in IEEE Transactions on Industry Applications, Vol. 1A-12, No. 6, November/December 1976. While the throughput and energy efficiency of such a plant is increased, the large pressure drop associated with the suspension preheater remains and there may be an associated equipment loss if the removed section of the long rotary kiln cannot be used for other purposes and is discarded. In addition, the large capital expense and installation cost associated with the multi-stage suspension preheater remains.
The present invention is a method of converting an existing long rotary kiln cement plant to a higher throughput, more energy efficient calcining furnace plant which overcomes the noted problems associated with use of the multi-stage suspension preheater.
The invention employs the concept of using a section of a rotary kiln as a preheater which feeds preheated raw material to a calcining furnace which in turn is connected to the input of a rotary kiln clinkering device, the output of which is further fed to a clinker cooler. The rotary preheater has a much lower pressure drop associated therewith and efficiently promotes heat exchange through internal lifters and chains to preheat the raw meal. The rotary preheater also has lower equipment and installation costs associated therewith as compared with multi-stage suspension preheaters. The use of a rotary preheater is particularly advantageous in the conversion of an existing long dry or wet process rotary kiln cement-making plant, as a center section of an existing long kiln can be cut and removed leaving a beginning portion of the kiln for use as the rotary preheater. The output of the rotary preheater is connected to a newly installed calcining furnace and an associated material separator. The remaining end portion of the existing kiln is connected to the output of the calcining furnace and the associated material separator for use as a clinkering kiln. The conversion of an existing kiln achieves the desirable objectives of a reduced energy consumption and increased throughput, while retaining use of a major portion of the original equipment.
These and other objects and advantages of the invention will become more apparent in the subsequent detailed description of the invention.